| dc.description.abstract | The use of GaN-based high electron mobility transistors (HEMTs) is inevitable in high power and high-frequency applications. Electron transport properties play a significant role in determining the RF and power switching performances. High electron mobility reduces the electron transit time between the drain and the source. Whereas, high electron density improves the transconductance and current density. Therefore, in terms of epitaxy, electron transport properties are paramount in designing state-of-the-art transistors. Although, AlGaN barrier heterostructure routinely exhibits electron mobility above 2100 cm2/V-s, the same is not so obvious in the indium containing heterostructures. The GaN channel, the heterointerface and the barrier, all contribute to electron scattering. Therefore, it is important to address each one of these to overcome the limited transport properties in indium containing heterostructures.
Furthermore, the scattering mechanisms in indium containing thin barrier heterostructures could be complex. Whereas, AlGaN/GaN heterostructures may be an excellent tool to study the heterointerface. Therefore, the heterointerface characteristics that influence the scattering of 2DEG, confined in the AlGaN/GaN quantum well, are first controlled by modulating the surface reconstruction of the intrinsic GaN channel under different in-situ growth conditions such as growing the GaN channel under either nitrogen or hydrogen ambient and with different Ga precursors such as trimethylgallium (TMG) and triethylgallium (TEG). The secondary ion mass spectroscopy (SIMS) depth profiles indicate that, by changing the carrier gas from H2 to N2 at the end of the thick GaN buffer, reduces the residual carbon concentration from 4x1017 cm-3 to ~2x1017 cm-3. The use of TEG resulted in further reduction of residual carbon concentration in the GaN channel down to 6.4x1016 cm-3. The mobility at 10K reaches up to 7900 cm2/V-s and 9360 cm2/V-s in samples where the GaN channel is grown with TMG and TEG respectively, both in H2 ambient. Further changing the carrier gas from H2 to N2, the mobility reaches up to 28000 cm2/V-s in the sample with TEG grown GaN channel. However, a 46% drop in 2DEG is also observed as compared to the samples grown in H2 ambient due to lower Al content in the AlGaN barrier. A systematic study of the scattering events reveals that a component of the Hall mobility, which ranges between 9.2x103 cm2/V-s to 3.4x104 cm2/V-s among the three samples is governed by unknown scattering events. This cannot be explained completely by considering all the conventional scattering mechanisms such as phonon-phonon scattering, interface roughness scattering, dislocation density scattering etc. An in-depth analysis reveals a significant scattering of channel electrons by the charged states at the GaN/AlN/AlGaN interface. The extracted interface states from the temperature-dependent Hall mobility indicates 29% and 80% reduction in charged states when the GaN channel/interface is grown using TEG in H2 and N2 ambient respectively as compared to the GaN channel grown with TMG in H2 ambient. Furthermore, the estimated interface states from the temperature-dependent subthreshold slops conducted on the fabricated high electron mobility transistors are in good agreement with the charged interface states extracted from the temperature-dependent Hall measurements and therefore, further supports the postulate. This also provides experimental evidence of electron scattering by the charged interface states in III-nitride heterostructures. A good understanding of this new scattering mechanism in GaN heterostructure may help in designing high-performance III-nitride devices in the future.
The main problem with the AlInN barrier is indium segregation due to the highest immiscibility. This results in compositional inhomogeneity within the barrier and often responsible for electrical performance degradation. It is known that AlGaN is the most stable alloy as compared to AlInN. Therefore, a combination of these two alloys may increase the thermodynamic stability in the quaternary AlInGaN alloy. Significant improvement in electron mobility has been reported by several authors in AlInGaN/GaN heterostructures by adding small amount of Ga to the AlInN alloy. However, design and epitaxy of a quaternary barrier for the high electron mobility transistors can be difficult. Moreover, there is no agreement on the composition of the material in the literature as opposed to the lattice-matched AlInN. There was no guideline as well on the design of this complex alloy. We propose a new mechanism where thermodynamic stability plays an important role in controlling the electron transport properties in these heterostructures. A quantitative investigation of the thermodynamic stability of AlInGaN barrier has been carried out analytically, for a wide range of compositions (0.5 ≤ Al ≤ 0.8, In = 0.2, 0.15, 0.1). A slow change in the thermodynamic stability is observed when the Ga atoms replace only the Al atoms. In contrast, a significant improvement in thermodynamic stability is observed, when the indium atoms are replaced by the Ga atoms in the same Al0.83In0.17N layer. It is found that Al content in the range of 65% to 70% with 10% indium exhibit the highest thermodynamic stability within the calculated composition range owing to the significant reduction in total elastic strain in the barrier. Thereby, it leads to the highest electron mobility, as evidenced by the experimental observations in this work, i.e. electron mobility of 2,090 cm2/V-s with a sheet carrier density of 1.09x1013 cm-2. Therefore, the thermodynamic stability apart from commonly observed scattering mechanisms may at least be partially held responsible for the consistent improvement in electron mobility in AlInGaN/GaN heterostructures.
The fabricated high electron mobility transistors out of these AlInGaN/AlN/GaN heterostructures, grown on 150 mm p-type low resistivity (resistivity~ 20-100 Ω-cm) silicon substrate demonstrate state-of-the-art Johnson’s figure-of-merit (JFOM). Current gain cut-off frequency (fT) of 83 GHz and 63 GHz and power gain cut-off frequency (fmax) of 95 GHz and 77 GHz with a three-terminal off-state breakdown voltage of 69 V and 127 V, resulting in a high JFOM of 5.7 THz-V and 8.1 THz-V are achieved on the devices with a gate length of 0.16 m and gate to drain distance of 2 m and 4 m, respectively. The analysis suggests that the high JFOM could be due to the superior electron transport properties and lower residual carbon concentration in the channel due to the use of TEG grown GaN channel in N2 ambient. 2D device simulation further shows that the trap density in the GaN channel should be in the lower order of 1016 cm-3 to avoid the effect of parasitic charge modulation on fT. The fT and J-FOM are comparable or better than the reported values obtained on high resistivity silicon and SiC substrates for devices with similar gate length. On the other hand, the GaN-on-Si HEMTs on the LR-Si substrate exhibit lower power gain and power-added efficiency due to strong capacitive coupling effects. TCAD large signal output power simulation indicates significant improvements in output power by minimizing the defects and free charge carriers in the GaN buffer even in the presence of the parasitic channel conduction and the conductive silicon substrate. We further propose a modified equivalent circuit model of the parasitic conduction to take into account the conductivity of the GaN and AlGaN buffer. Therefore, this study shows a new viewpoint on realizing mm-wave GaN HEMTs on low resistivity silicon substrate for next-generation transistors. | en_US |